The present invention relates generally to a self-aligned mask formed utilizing differential oxidation rates of different materials, and more particularly to a method for forming a self-aligned oxide mask on a polycrystal field of a base layer of a CVD grown base NPN transistor wherein the base layer compromises single crystal Si (or Si/SiGe-which means a layer of single crystal silicon on top of a layer of single crystal silicon germanium) at active area and polycrystal Si (or Si/SiGe-which means a layer of polycrystal silicon on top of a layer of polycrystal silicon germanium) on the field isolation.